1. Field of the Invention
The present invention relates to the cracking of hydrocarbon feed using water as a supplement to or substitute for dilution steam.
2. Description of Background
Steam cracking has long been used to crack various hydrocarbon feeds into olefins. Conventional steam cracking utilizes a pyrolysis furnace, which has two main sections: a convection section and a radiant section reaction zone. The hydrocarbon feed typically enters the convection section of the furnace as a liquid (except for light feeds which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by mixing with steam. The vaporized feed and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, such as quenching.
By way of non-limiting illustration, in a typical pyrolysis reactor furnace for the production of ethylene from naphtha feed, the hydrocarbon feed enters the convection section of the furnace where it is preheated in first heat exchange tubes by indirect contact with furnace flue gas from the radiant section. A dilution steam stream can enter the convection section wherein it is superheated, also in heat exchange tubes by indirect contact with furnace flue gas from the radiant section. The superheated dilution steam is then mixed with the hydrocarbon feed to reduce the hydrocarbon partial pressure in the radiant section reaction zone of the furnace. It is well known in the art that reducing the hydrocarbon partial pressure in the reaction zone (1) increases the selectivity of the reactor to desired olefinic products such as ethylene, and (2) reduces the rate at which undesirable coke is formed and deposited on the interior surfaces of radiant section tubes. The superheated steam is mixed with the preheated hydrocarbon feed producing a vapor hydrocarbon/steam mixture which is further preheated to a temperature suitable for conveying the mixture to the radiant section of the furnace. The cracking reactions which produce the desired ethylene product and other byproducts take place predominantly in the radiant section of the furnace. After leaving the radiant section, the reactor effluent is rapidly quenched in a quench system to stop the cracking reactions.
For well-known energy efficiency purposes, it is desirable to recover as much heat as possible from the flue gas leaving the radiant section and flowing through the convection section of the furnace to the furnace flue gas exhaust. Thus, hydrocarbon feed and dilution steam are heated in the convection section, typically by indirect contact with flue gas from the radiant section. Other recovery services may also be included in the convection section such as a boiler feed water preheater and/or a steam superheater used to superheat high pressure steam which may be generated in the quench system of the furnace.
In some furnace designs, boiler feed water preheat and/or high pressure steam superheat services may not be available to absorb heat from the flue-gas stream flowing through the convection section. In such cases, the flue gas may exit the furnace at unacceptably high temperatures, for example, as high as 600–700° F. This represents a substantial energy inefficiency, as some designs provide for flue-gas discharge temperatures as low as, for example, 250–300° F.
In other instances, it may be desirable to provide additional dilution steam to further decrease the hydrocarbon feed partial pressure. But such steam may not be available at reasonable cost.
The present invention provides an advantage of providing for additional dilution steam when it is otherwise unavailable at a reasonable cost.
The present invention also provides another advantage of improving furnace energy efficiency. These and other features and advantages of the present invention will become apparent from the following description and claims.
Separate applications, one entitled “CONVERTING MIST FLOW TO ANNULAR FLOW IN THERMAL CRACKING APPLICATION,” U.S. application Ser. No. 10/189,168, Family Number 2002B064, filed Jul. 3, 2002, and one entitled “PROCESS FOR STEAM CRACKING HEAVY HYDROCARBON FEEDSTOCKS”, U.S. application Ser. No. 10/188,461, Family Number 2002B063, filed Jul. 3, 2002, are being concurrently filed herewith and are incorporated herein by reference.